1. Field of the Invention
The present invention relates to an apparatus for generating plasma utilizing electron cyclotron resonance (ECR)
2. Description of the Related Art
In the field of manufacture of semiconductors, etc., there is known and practiced an apparatus in which ECR is developed in a rarefied gas to generate plasma while electromagnetic waves are supplied to a resonance magnetic field. An improvement of such an ECR type plasma generating apparatus has been proposed which employs a permanent magnet as a magnet for generating the resonance magnetic field, and is installed in a vacuum to perform the plasma CVD (chemical vapor deposition) (see Japanese Patent No. 2,011,461, "Plasma Generating Apparatus"). This improved plasma generating apparatus has such advantages that the structure is simple and compact, and plasma can be locally produced with high flexibility of design. However, when the apparatus including a permanent magnet is arranged in a vacuum, the following problems occur. If the permanent magnet is made of a porous material or a material containing an organic binder, gas is vigorously released from the material upon heating in the plasma generating process. Magnetic powder is sputtered from the permanent magnet into the vacuum. Further, it is difficult to cool the permanent magnet.
Those problems occurred with arrangement of a permanent magnet in a vacuum have been overcome by another improvement of the ECR type plasma generating apparatus which comprises a vacuum vessel capable of maintaining a vacuum state therein, an antenna for radiating electromagnetic waves into the vacuum vessel, apermanent magnet for forming a resonance magnetic field, and an outer sleeve made of an insulator and accommodating the permanent magnet therein, the outer sleeve being permeable to lines of magnetic force and having air tightness (see Japanese Patent No. 2,080,292, "Plasma Generating Apparatus Utilizing Electron Cyclotron Resonance").
However, the plasma generating apparatus employing the permanent magnet, disclosed in the above Japanese Patents, still have such a disadvantage that a produced resonance region is limited. More specifically, as well known, both N and S poles always exist in the same solid body of a permanent magnet, and lines of magnetic force generate from the N pole and terminate at the S pole. Because of this specific property, the permanent magnet cannot distribute a resonance magnetic field over a so wide space as covered by lines of electric force, thus resulting in a restricted region where plasma can be produced.